Many types of industrial processes are monitored using industrial sensing probes. Such probes are available in standard sizes. A typical probe comprises a sealed connection head that includes an opening for the extension of a probe sensor into the machine and a connection head internal volume in which a conversion circuit for driving the sensor is encased and protected from the environment. The probe can communicate with a remote controller via a two-wire 4–20 mA current loop (the conversion circuit is also known as a two-wire transmitter), or via data link.
In operation, the sensor senses a machine parameter such as temperature or pressure. The conversion circuit drives the sensor to generate a raw sensor signal indicating the sensed parameter and converts this signal into a DC signal that meets the 4–20 mA standard. It should be noted that the same DC link is typically used for both power input and signal output. In some implementations, the link is also used for programming the conversion circuit.
In the industry, the conversion circuit and the sensor are considered an integral unit with the rest of the probe. Thus, they are connected to each other (and to the external link) via screw terminals. If the circuit or the probe needs to be replaced, a skilled person is required to open the terminals and ensure correct matching of the wires to the terminals on the circuit. Periodic testing of all the conversion circuits in a factory is thus often a particular laborious activity.
In some cases, users are interested in reading the measurements at a machine. As the probe and circuit are considered integral, one solution is to manufacture the circuit with an external plug for attaching an add-on display to the probe. Another, well known, solution is to provide a separate display unit that sits on the DC link and is powered by the link. Being separate, this display unit requires separate circuitry and is typically as expensive as a conversion circuit. In addition, when the conversion circuit is calibrated, re-programmed or used for a different type of sensor, also the display needs to be adjusted.